1. Field of the Invention
This invention relates generally to the field of power splitters, and, more particularly, to a planar power splitter.
2. Description of the Related Art
Power amplifier balancing is a well-known and established method to distribute a varying load of different channels equally among a single amplifying element. Commonly available 3 dB hybrid devices or other types of coupler elements are used to split radio frequency (“RF”) signals into a plurality of components prior to amplification and to combine the components after they have been amplified. This splitting, amplifying, and combining operation takes advantage of coherent superposition on the coupler's output ports, which may lead to the cancellation of most components, and constructive interference for only one of the signal channels.
A signal applied to one input port of the coupler element will travel different paths inside the coupler element. The different paths subject the signal to different phase changes along the different paths, which can result in a total cancellation at the other input ports and/or a partial constructive superposition on the output ports. In a balanced element, the input power may be distributed equally among the output ports, but high isolation is maintained between all input ports with a low input reflection. The operation complimentary to splitting a signal is the combining of signal components and providing each component at a single output port. The combining operation is made possible by injecting the single components in a well-defined phase state and amplitude into the input ports of a coupler element. Due to the same physical mechanism as used for the equal splitting, the injected components may appear on a single output port. Additionally, a plurality of signals from different coherent sources may be superimposed. Typically, multi-port combiners may be constructed by combining multiple (e.g., 3 dB) hybrid devices to form a network structure, commonly referred to as a Butler matrix. Butler matrices based on a 2-way combiner may therefore have a 1:2n splitting ratio, where n is a positive integer resulting in 2n input and 2n output ports per network.
In certain communication systems, such as a personal communications service (PCS) system having 3-sector or 6-sector cells, a different number of ports may be required (i.e., 3 or 6). Accordingly, the design of the network is not readily implemented using a regular 2n Butler matrix. Commercially available devices for implementing such networks have significant disadvantages. For example, commercially available combiners are either very large with a medium range insertion loss (e.g., about 0.5 dB) or they may be comparably small but have an increased insertion loss (e.g., about 0.9 dB). Moreover, the commercially available devices show a port isolation not better than −20 dB. These limitations can lead to increased crosstalk between adjacent sectors, thus degrading the system capacity due to an increased interference level.
Couplers having an odd number of ports (e.g, 1×3 splitters, 3×3 and 5×5 couplers) have been proposed and fabricated. However, these couplers have been formed using three-dimensional or multi-layer architectures. Three-dimensional couplers may be difficult or impossible to integrate into other devices formed in or on semiconductor chips. Although multilayer couplers may be incorporated into devices formed in or on semiconductor chips, the difficulty and expense of fabricating a multilayer coupler typically increases in proportion to the number of layers used to form the multilayer coupler. A planar implementation of a 3×3 coupler has been proposed that includes two concentric rings connected at six locations. The outer ring is larger that the inner ring by multiple wavelengths, e.g., multiple sections having electrical lengths of 360°. However, the additional electrical length of the outer ring reduces the operational bandwidth of the coupler.